Fluorine-containing copolymers are excellent in properties such as heat resistance, chemical resistance, weather resistance, and stain resistance, and are used in various fields such as semiconductors, automobiles, architecture, electrics and electronics, chemical plants, and medicine-related industries.
Various methods of further improving the properties, such as heat resistance, mechanical properties, and radiation resistance, of the fluorine-containing copolymers are studied.
One known method of modifying fluorine-containing copolymers is irradiation. Commonly known examples of such a modifying method include a method in which a fluorine-containing copolymer is heated up to a temperature of not lower than the melting point thereof and then the copolymer is irradiated with radiation (Patent Literature documents 1 and 2).
If a molded article of a fluorine-containing copolymer is heated up to a temperature of not lower than the melting point of the fluorine-containing copolymer and is irradiated with radiation, the molded article is deformed unfortunately. Further, irradiation causes great deterioration of the fluororesin, failing to sufficiently provide desired mechanical properties.
Patent Literature 3 discloses a method of producing a modified fluororesin without pre-heating, including irradiating the resin with ionizing radiation having a dose rate as high as 100 kGy/sec or higher from a particle accelerator at a predetermined exposure of 200 kGy to 100 MGy to crosslink the resin, thereby easily improving the heat resistance and the chemical resistance in a short time.
Patent Literature 4 discloses that the heat-aging resistance and compression-set resistance are improved by irradiating a fluororesin heated up to 0° C. to 150° C. or up to 0° C. to the crystal dispersion temperature thereof with ionizing radiation at an exposure of 5 Gy to 500 kGy and maintaining the irradiated fluororesin at a predetermined temperature for a predetermined period of time.
Patent Literature documents 5 to 7 disclose that a tetrafluoroethylene/perfluoro(alkyl vinyl ether) copolymer having a heat of crystal fusion within a specific range or a composition containing the copolymer is irradiated with ionizing radiation of 10 kGy or higher at an irradiating condition of 100° C. or lower.
Patent Literature 8 discloses a method of producing a modified fluororesin molded article including, for example, applying a fluororesin onto a metal substrate, which is less likely to bond to the fluororesin, irradiating the fluororesin with ionizing radiation at 200° C. to 400° C. to crosslink the fluororesin, and peeling or separating the fluororesin from the substrate.
Patent Literature 9 discloses a method of producing a composite material including a crosslinked fluororesin layer having excellent abrasion resistance and adhesion with a substrate, including the steps of: forming a fluororesin layer on a substrate; heating the fluororesin layer up to a temperature within the range from the melting point of the fluororesin to the temperature 150° C. higher than the melting point and sintering the fluororesin layer; adjusting the temperature of the sintered non-crosslinked fluororesin layer to a temperature within the range from the temperature 60° C. lower than the melting point (Tm) of the fluororesin to the temperature 1° C. lower than the melting point; and irradiating the fluororesin with radiation to crosslink the fluororesin.
Patent Literature 10 discloses a material covered with a modified fluororesin including a crosslinked fluororesin film covering a substrate having thermal stability at a temperature of not lower than the melting point of the fluororesin, wherein the fluororesin is crosslinked by ionizing radiation at a temperature within the range from 250° C. to 400° C.